Microporous membranes can be used as battery separator film (“BSF”) in, e.g., primary and secondary lithium batteries, lithium polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc secondary batteries, etc. When microporous polyolefin membranes are used for battery separators, particularly lithium ion battery separators, the membranes' characteristics significantly affect the properties, productivity and performance of the batteries. Accordingly, it is desirable for the microporous membrane to have a relatively high meltdown temperature and be electrochemically stable under conditions of battery manufacturing and use, particularly in batteries subjected to relatively high temperatures as can occur under overcharge or rapid-discharge conditions.
Electrochemically stable microporous membranes having a relatively high meltdown temperature have been produced from polypropylene. For example, Japanese Patent Application JP 10-279718 discloses multi-layer microporous membranes having outer layers comprising polypropylene, the polypropylene content of the outer layers being in the range of 80 wt. % to 100 wt. %. While these membranes have a relatively high meltdown temperature and electrochemical stability, the large amount of polypropylene in the outer layer undesirably lessens the membrane's electrolyte affinity and increases the membrane's moisture retention.
There is therefore a need for microporous membranes having an improved balance of electrochemical stability, high meltdown temperature, high electrolyte affinity, and low moisture retention.